Disordered metamaterial coating for daytime passive radiative cooling

Abstract

In this theoretical study, a disordered metamaterial coating with randomly embedded TiO2 dielectric microspheres in a polydimethylsiloxane matrix has been designed for the purpose of daytime passive radiative cooling. While retaining the necessary optical properties of high reflectivity (≈94%) in the solar spectrum and high emissivity (≈96%) in the atmospheric transparency window, the coating exhibits the following additional desirable properties: (a) low volume fraction of TiO2 microspheres, ensuring minimal possibility of agglomeration of particles during fabrication; and (b) a cooling power of 81.8 W/m2, which is among the highest for similar coatings that have been developed. We also show how a modified form of Kubelka-Munk theory with empirical relations originally developed to analyze optical scattering in biological tissue layers can be used for designing radiative cooling structures. The predictions from this method have been validated using Monte Carlo simulations. It is expected that this study will motivate further similar designs in the rapidly expanding market for effective and easy-to-fabricate coatings for daytime passive radiative cooling applications.